1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital color copying machine.
2. Description of the Prior Art
In a digital color copying machine, a document is read to obtain digital data of red, green and blue. The digital data are converted to image data of cyan, magenta, yellow and black to be reproduced on a sheet of paper. The digital data are subjected to various processing such as shading correction, density conversion, color correction, edge emphasis, smoothing, gamma correction and the like.
Prescan of the document is performed for determining the conditions on forming an image of the digital data in a sheet of paper. Data obtained with a prescan on a document to be reproduced is performed for detection of document size, shading correction or the like. For example, automatic exposure is performed on prescan data for correcting image data to reproduce background of a document as white. However, the automatic exposure processing is effective only for a monochromatic character image because a reproduced image becomes dull for a color image or a photograph image. It is desirable that automatic exposure processing is performed for a color image or the like.
Auto color selection is also performed on prescan data to determine if a document is a monochromatic image or a full color image. Image data on the document is processed according to the auto color selection.
If the automatic exposure processing and the auto color selection are performed at the same time, a document size has to be detected beforehand. Then, prescan data are used twice. However, it is not desirable to perform prescan twice because it takes a longer time for copying.
A digital copying machine reads a document to get digital data of red, green and blue thereof and converts them to image data of cyan, magenta, yellow and black, which are reproduced on a sheet of paper with toners having the four colors. The image data are subjected to MTF correction before printing. For a document image comprising black characters, it is desirable to emphasize edges of black characters. Then, for example, black characters are detected in a document image, and the image data for the black characters are subjected to edge emphasis and are reproduced with black toners. Thus, the black characters are reproduced with black toners, and the image quality is improved.
As to the black characters, image data of the cyan, magenta and yellow are suppressed at the edges according to brightness component of the image data. However, if black characters are described in a colored background, an amount of suppression of the image data of the cyan, magenta and yellow becomes large, so that areas adjacent to the black characters become white though they have a color of the colored background. This deteriorates image quality in a reproduced image.
Black characters are detected as black image in an area between a pair of edges. However, for characters of dark blue, dark green or the like, edges are liable to be decided erroneously. Further, for an image comprising black dots, edges are decided erroneously, a Moire pattern may happen. It is also a problem that in an electrophotographic copying machine, toners are liable to have a higher density at a leading edge and a lower density at a trailing edge, and this also deteriorates image quality of black characters.
A digital copying machine can control image qualities on various items such as hue, chroma, color, balance and the like besides gamma correction and edge emphasis. When a user wants a copy of a desired image quality, he or she adjusts the various items before starting copying operation. If the reproduced image does not have the desired image quality, he or she has to adjust the various items again based on the reproduced image. However, because a number of items to be adjusted is large, it is difficult to predict an image quality. Then, it is desirable to adjust image quality efficiently.
As to color balance, density level of toners of cyan, magenta, yellow and blue is controlled independently of each other. Then, density on a sheet of paper is also affected by adjustment of the color balance. Then, a total image density of a reproduced image may become higher or lower, and an amount of toners fixed on paper becomes uneven. It is also a problem that jam or the like may happen.